1. Field of the Invention
The invention relates generally to the structure and fabrication of integrated circuits, and more particularly to integrated circuits having metal oxide semiconductor field effect transistors (MOSFETs) with multi-layer metal gate electrodes.
2. Background
Advances in semiconductor manufacturing technology have led to the integration of tens, and more recently hundreds, of millions of circuit elements, such as transistors, on a single integrated circuit (IC). To achieve such dramatic increases in the density of circuit components has required semiconductor manufacturers to scale down the physical dimensions of the circuit elements, as well as the interconnection structures used to connect the circuit elements into functional circuitry.
One consequence of scaling down the physical dimensions of circuit elements has been a dramatic increase in the complexity of transistor engineering for the process engineers and scientists that develop advanced microelectronic manufacturing processes. In the past it was common to consider gate oxide thickness, polysilicon line width, source/drain junction depth and lateral diffusion therefrom, and some threshold adjusting ion implant as the primary parameters of transistor design. However as transistor dimensions entered the deep submicron region, the transistor structure became more complex with the inclusion of elements such as graded well doping, epitaxial wafers, halo implants, tip implants, lightly doped drain structures, multiple implants for source/drain formation, silicidation of gates and source/drains, and multiple sidewall spacers, among others. Because of the complex nature of deep submicron transistors, workers in this field tend to characterize, or specify, transistors not so much by physical dimension (e.g., gate length) but rather by electrical characteristics such as drive current (Ion) versus leakage current (Ioff).
For high speed operation of microelectronic components, circuit designers prefer transistors tuned for high drive currents. On the other hand, low leakage, i.e., low off-state current, is preferred so as to reduce power consumption. Typically, the structural and doping parameters that tend to provide the desired increase in drive current also tend to provide an undesired increase in leakage current. Use of metal gates will tend to improve drive current performance by reducing or eliminating the poly depletion effect. Unfortunately, many metals, while reducing or eliminating the poly depletion effect when used to form a gate electrode, will undesirably result in increased leakage current because the work function of that metal gate is not appropriate for providing the desired transistor threshold voltage.
What is needed are structures and methods for obtaining the desired electrical performance from insulated gate field effect transistors.
Additionally what is needed are structures suited for complementary metal oxide semiconductor (CMOS) integrated circuits, and methods of making such integrated circuits that are suitable for integration into a manufacturing process.